external trigger analog input 6015

[jtaki]

Thanks Brad.

Here is another issue i had:

i modified the example program "AcqVoltageSamples_IntClk" to have three channels to read from.  i found that if i create the channels in this order, the data is correct:

// Create a channel

myTask.AIChannels.CreateVoltageChannel("Dev1/ai0", "", AITerminalConfiguration.Nrse, Convert.ToDouble(0), Convert.ToDouble(10), AIVoltageUnits.Volts);

myTask.AIChannels.CreateVoltageChannel("Dev1/ai1", "", AITerminalConfiguration.Nrse, Convert.ToDouble(-5), Convert.ToDouble(5), AIVoltageUnits.Volts);myTask.AIChannels.CreateVoltageChannel(

"Dev1/ai2", "", AITerminalConfiguration.Nrse, Convert.ToDouble(-5), Convert.ToDouble(5), AIVoltageUnits.Volts);

however, if i create them in this order (the only change):

// Create a channel

myTask.AIChannels.CreateVoltageChannel("Dev1/ai2", "", AITerminalConfiguration.Nrse, Convert.ToDouble(-5), Convert.ToDouble(5), AIVoltageUnits.Volts);

myTask.AIChannels.CreateVoltageChannel("Dev1/ai1", "", AITerminalConfiguration.Nrse, Convert.ToDouble(-5), Convert.ToDouble(5), AIVoltageUnits.Volts);myTask.AIChannels.CreateVoltageChannel(

"Dev1/ai0", "", AITerminalConfiguration.Nrse, Convert.ToDouble(0), Convert.ToDouble(10), AIVoltageUnits.Volts);

then the data i get for my channel "Dev1/ai2" is incorrect.  specifically, if i read 4 samples, it would read the following for the following samples:

-3.09

-3.56

-3.92

-4.14

when i create the channels in the first order, the data is correct (verified with my scope)

-3.09

-3.09

-3.09

-3.09

Now, if in the first case, where the voltages are incorrect, i read these using a rate of 100,000.  if i lower this to 1000, then the results are more correct, but still not exact.  Am i oversampling, but is dependant on the order of creating the channels?

i am actually going to switch to a PCI 6122, thus maybe this wouldnt be an issue due to the 4 A/D channels??

1. any ideas on why the order of creating the channels for the 6015/6016 matters?

2. will it matter for the 6122?

thanks,

Joe

[jtaki]

Actually, i have now found that it is not really the order, but only the last one created works correctly.  i applied a voltage to the other two channels, and the first and second channel always have the problem, but the third channel is correct.  i can mix up the order in any way and it always works that the last one is the only correct one...

???/

Joe

[Brad_K]

Hi Joe,

The sample rate and channel order dependence make it sound like you may be seeing ghosting. How Do I Eliminate Ghosting From My Measurements? has more information on what causes this problem and how to resolve it.

Ghosting is not an issue for the PCI-6122 due to its per-channel ADCs.

Brad

[jtaki]

that sounds like exactly it.  thanks.

i will lower my sample rate until i get my 6122.

Joe